As is known, spinal fusions can be utilized when there is a deformity of the spine, instability of the spine, a damaged intervertebral disk, trauma to the spine, a tumor on the spine, spinal pain or infection or degeneration of the spine. A number of posterior spinal fixation methods are available for stabilizing the spine and permitting fusion to occur. Commonly, these devices are adapted for use with posterior spinal screws that are inserted into the vertebrae.
A tulip is often provided on the posterior end of the posterior spinal screw for coupling the screw to a transverse rod that extends vertically along the spine. In this regard, the screw head is positioned within a recess formed at one end of a cylindrical-type tulip body. The rod sits in a recess at the other end of the cylindrical body. The recess is provided at the top end of the body and extends perpendicular to the longitudinal axis of the tulip so that the rod may extend perpendicular to the axis of the screw. A rod capturing screw may also be provided on the tulip. The vertical rod is coupled to one or more other screws, by means of tulips or otherwise, thus connecting several vertebral bodies together. Screws and rod serve as a mechanism to maintain the vertebral bodies in a generally fixed position relative to each other while they are fused together by a bone graft.
Other mechanisms exist for securing together posterior spinal screws. For example, a rigid single structure may be used. In one example of this embodiment, the a screw itself may have a recess extending transversely or perpendicular to the longitudinal axis of the screw, which recess is formed integral with the proximal or posterior end of the screw. The rod is seated in that recess, and is secured within the recess by a rod capturing screw or other suitable means. Other mechanisms include a side loading assembly. In one example of this embodiment, a securement mechanism sits over the posterior end of the screw and has a through-hole spaced from and perpendicular to the longitudinal axis of the screw. The rod is threaded through the securement mechanism. Alternatively, the rod may be threaded through a recess within the screw and which extends perpendicular to the screw, but is spaced or positioned along the longitudinal axis of the screw.
The rod may be pre-threaded through each of the various securement mechanisms. Once the screws are placed within the vertebrae, the securement mechanisms are placed over the respective screws. Alternatively, the securement mechanisms may be placed on the posterior end of the screws. Then the construct is placed within the recesses in the securement mechanisms.
Unfortunately, problems exist with each of the aforementioned systems. For instance, with respect to the tulip design, the rod sits posteriorly of the screw head. Thus, a pedicle or other posterior spinal screw having a tulip on top of it increases the distance between the pedicle screw head and the top of the mechanism that askews a rod, causing prominence of the total construct. Additionally, only a limited amount of rotation is permitted due to the geometry of currently used tulip devices. Namely the tulip can only rotate about a cone having a conical axis of thirty degrees from the central axis of the screw head. In order to load the rod into the tulip, or slot in the screw, if the rod is not directly in line with the slot, or on top of the tulip, or within the thirty degree cone of that tulip and the head of the screw, the physician must use a tool called a “persuader” in order to push or force the rod into the tulip. This causes preload on the pedicle screws within the pedicle. Moreover, a torquing of the pedicle may result as the physician forces the rod capturing screw on top of the tulip or screw, which then causes torsional stresses onto the pedicle screw. In order to avoid this, a physician must use a special tool that does not allow the torque to occur.
With respect to the side loading system, only one pivotal direction of the rod capturing mechanism exists for an embodiment having one opening. As a result, a physician must preload all the assemblies onto the rod first, and then where applicable, slide the preloaded assembly on to the screw that has already been placed in the body. In other words, a physician must preload the rod, and place the assemblies on the screw. Unfortunately, the physician must still persuade the rod into a side loading mechanism in order to make sure that it is exactly at the right level and in the anterior or posterior direction.
An additional disadvantage is that if a physician uses a facet screw or a laminopedicle screw, in which the screw is placed into the spine at a sharp angle, usually around forty-five to sixty degrees relative to the axis of the spine, or relative to the horizontal plane relative to the spine, tulips cannot be used. The cone of motion, or the polyaxiality of the tulip is only thirty degrees. Additionally, the tulip would be incredibly prominent if a facet screw or laminopedicle screw is used.
Accordingly, what is needed in the art is an improved system and device for stabilizing adjacent vertebra in the mammalian spine. More particularly, what is needed in the art is a low profile, non-prominent spinal connection assembly that allows motion of the spinal connection assembly relative to the head of the screw to which it is attached to be able to pivot in a large degree of motion in at least one plane, and preferably a large degree of motion in at least two planes, so as to be capable of pivoting about the axis of the screw head and thus decrease the pre-load and torque on the screw.